Quadrupole mass filters are used as part of a mass spectrometer to analyse the chemical composition of an ionised sample of material. A QMF uses an electrical field to filter the ionised particles according to their mass-to-charge ratios. The electric field is controlled by adjusting the AC and DC voltages so that only ions of a specific mass-to-charge ratio can translate axially through the QMF. The applied voltages have a directly proportional relationship to the mass-to-charge ratio of the transmitted ions, hence the magnitude of the applied AC and DC voltages determines which ions can travel through the QMF. Thus, a perfect QMF will only transmit ions of a particular mass-to-charge ratio when particular AC and DC voltages (nominal voltages) are applied. Unfortunately many QMFs are far from perfect and erroneously transmit ions of a specific mass-to-charge ratio at the wrong AC and DC voltages. A quadrupole mass filter is said to have a precursor fault if it transmits ions of a particular mass-to-charge ratio at voltages that are lower than the nominal AC and DC voltages.
A conventional quadrupole mass filter (1) is depicted in FIG. 1. The QMF includes four electrode rods (2) held in a square parallel array. The electrode rods are arranged as opposing pairs around an origin along an x-axis and a y-axis and they extend substantially parallel to a z-axis (not shown). Each electrode rod has a substantially circular cross-section with a radius r. The electrode rods are positioned on a pitch circle diameter (PCD) (3). The inscribed circle has a radius, r0 (4).
The electrode rods are connected to a combined AC and DC power supply (5). Both an AC and a DC voltage are applied to each electrode. FIG. 1 shows that the pairs of opposing electrodes are electrically connected together. Thus, the opposing electrodes in the x-axis have a common power supply and the opposing electrode rods in the y-axis have a common power supply. Consequently, there is no potential difference across the x-axis electrode rods or across the y-axis electrode rods. An electrical field is generated around each electrode rod when they are excited. In a perfect QMF the electrical fields of each of the four electrode rods combine to create an electric field that approximates to a rectangular hyperbolic electrical field. The effective part of this resulting electrical field is generated in a central void formed within the square array of the electrode rods.
The ions have complex trajectories within the QMF. The QMF filters the ions by using an electric field to control the motion of ions with a specific mass-to-charge ratio. This is achieved by applying AC and DC voltages to the electrode rods so that an electrical field is set-up to stabilise the motion of certain ions in the x and y directions. As mentioned above, the magnitude of the applied voltages is directly proportional to the mass-to-charge ratios of the ions. Consequently, in a perfect QMF, particular AC and DC voltages set-up an electrical field that can only stabilise the motion of ions with a particular mass-to-charge ratio. The electrical field enables the ions of the specific mass-to-charge ratio to oscillate in a stable manner in the x and y-axes whilst moving along the z-axis. As a result, these ions are able to travel through the QMF. Ions with a different mass oscillate in an unstable manner within the QMF. The trajectory of these ions grows until they strike an electrode rod and are lost.
The electrical field in the x-axis stabilises the trajectory of heavier ions, whereas the lighter ions are unaffected by the field and have unstable trajectories. Conversely, the electrical field in the y-axis stabilises the trajectories of lighter ions, whereas the heavier ions are unaffected by the field and have unstable trajectories. Consequently, the combined effect of the electrical fields in both axes determines the band pass mass filtering action of the QMF.
If a QMF has a precursor fault then the ions that should have unstable trajectories in the y-axis have, in fact, stable trajectories. Accordingly, they are able to travel through the QMF.
When positive ions are analysed, a positive DC potential is applied to the electrode rods in the x-axis and a negative DC potential is applied to the electrode rods in the y-axis. When negative ions are analysed, a negative DC potential is applied to the electrode rods in the x-axis and a positive DC potential is applied to the electrode rods in the y-axis.
A detector (not shown) is arranged to detect the transmitted ions as they exit the QMF. The detector is electrically connected to a data processing means (not shown) to generate a mass spectrum of the transmitted ions and display means (not shown) to display the mass spectrum. A mass spectrum indicates the ions that are transmitted as different AC and DC voltages are applied. The data processing means and display means are typically provided by a personal computer. The mass spectrum is usually displayed in graphical form.
A QMF may be used as part of a mass spectrometer to determine the different components of an ionised sample of material. This is achieved by varying the applied AC and DC voltages so that ions of many different mass-to-charge ratios can be sequentially detected. A QMF usually increases the voltages so that it scans up the ion mass-to-charge ratio scale. The detected ions are indicative of the composition of the sample of material.
FIG. 2 depicts a mass spectrum of perfluorotributylamine (PFTBA) which has been analysed using a QMF with a precursor fault. The sample of ionised PFTBA is analysed to detect ions with a mass-to-charge ratio of 69 Da. The presence of these ions is indicated in a parent peak (6). If the QMF were perfect, then the mass spectrum would only include the parent peak. However, FIG. 2 shows that the mass spectrum includes a precursor peak (7). The precursor peak indicates that ions with a mass-to-charge ratio of 69 Da have been erroneously transmitted at lower than nominal voltages.
The most common type of fault in a QMF is a precursor fault.
Precursor faults are extremely problematic because they affect the filtering action of the QMF. Consequently, a QMF with a precursor fault cannot be relied upon to provide an accurate analysis of the chemical composition of an ionised sample of material.
Quadrupole mass filters are very difficult to build. The manufacturing process is time consuming, expensive and requires precision engineering. Hence, the chances of incurring a precursor fault during the manufacturing process are high.
Up until now, the only way of dealing with a precursor fault is to strip, clean and rebuild a QMF. This is both time consuming, expensive and, again, requires precision engineering. Furthermore, there is no guarantee of success. As indicated above, it is very difficult to ensure the electrode rods are perfectly aligned and clean.
QMFs are often required to work at high resolutions. The resolution of the QMF, its ability to distinguish between adjacent ion masses, is set by controlling the U/V ratio. The precursor fault becomes increasingly apparent as the QMF scans up the mass-to-charge ratio scale. Thus, when a QMF with a precursor fault operates at a high resolution the filtering action is inaccurate.